Abstract

Laser speckle produced from a diffuse object can be used in
determining the angular position of a rotating object. When the
object rotates the backscattered speckle pattern, which changes
continuously but repeats exactly with every revolution, is sampled by a
suitably positioned photodetector. The photodetector output signal
is periodic, and one period is stored in the memory as a
reference. Shaft position can then be determined by the comparison
of this stored reference signal with the current photodetector output
signal. When the shaft is axially displaced, for example, by
vibration, the backscattered speckle pattern changes on the
photodetector and the similarity between the reference signal and the
current signal is reduced. We examine the cross correlation of the
real-time photodetector output signal and the stored reference signal
as a function of axial shaft position. Use of a rotating shaft when
collecting data is shown to be an efficient means by which to make
effectively several thousand independent estimates of the maximum axial
displacement tolerable before decorrelation of the photodetector
output. Theoretical results and experiments conducted show that the
decorrelation displacement varies, according to optical configuration,
to a maximum value of 0.7 of the beam diameter. This has important
implications for a proposed laser torquemeter as well as additional
applications in which changes to the sampled speckle pattern, including
decorrelation, are either desirable or undesirable.

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